Traditionally, the majority of general purpose road vehicles are powered by liquid fuels such as gasoline or diesel fuel. When the vehicle needs power, an internal combustion engine converts the chemical energy in the fuel into mechanical energy and a powertrain delivers that mechanical energy to vehicle wheels. The vehicles are used over a wide range of vehicle speeds, including both forward and reverse movement. Some types of internal combustion engines, however, are capable of operating efficiently only within a narrow range of speeds. Consequently, the powertrain typically includes a variable speed ratio transmission. Also, a differential assembly may connect the transmission output shaft to the vehicle wheels, providing an additional fixed speed ratio and permitting the left and right wheel to rotate at slightly different speeds as the vehicle turns.
In an effort to reduce the consumption of liquid fuel, some vehicles, called hybrid electric vehicles, utilize electrical energy storage such as a battery. The energy storage capability provides flexibility to perform the conversion of chemical energy when the conversion can be done most efficiently as opposed to always performing the conversion at the moment the power is demanded. Some hybrid electric vehicles, called plug-in hybrid electric vehicles, are also adapted to receive power directly in electrical form.